1. Field of the Invention
The present invention relates to electro-pneumatic converters.
2. Description of the Prior Art
A conventional electro-pneumatic pressure converter converts an electrical input signal to a nozzle back pressure, thereby obtaining a pneumatic pressure signal. For example, a piezoelectric element can be used for a nozzle flapper. The nozzle flapper is arranged to abut against a nozzle port and is shifted in accordance with an electrical input signal, thereby controlling an opening of a gap between the nozzle port and the nozzle flapper and changing the nozzle back pressure. Alternatively, the nozzle flapper can be vibrated at a frequency near a resonance frequency thereof. A ratio of a full-open time to a full-close time for a vibration cycle is controlled in accordance with an electrical input signal utilizing a preselected gap between the nozzle flapper and the nozzle port, thereby also changing a nozzle back pressure. In either converter described above, since an electrical input signal is converted to a nozzle back pressure, it is difficult to obtain a sufficient gain for an input signal. When such an electro-pneumatic pressure converter is operated in a two-wire control system using a signal of 4 to 20 mA, a power supply voltage for the converter must be obtained from the external input signal of 4 to 20 mA. However, if the input voltage is decreased for any reason and a predetermined voltage cannot be obtained, the control of a converter switching valve is impaired, and an abnormal output may be generated.